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wp_enqueue_script is a wordpress function that lets you add javascript files to your site. Just put it in your themes functions.php file or a plugin file. The great thing about it is that it looks if that file has been included already. If it hasn’t only then does it load it. You can also specify dependencies, i.e. jQuery. This means that it will always load the dependencies first. You can also specify if you want to load the script in the header or the footer. Here’s the function:

wp_enqueue_script( $handle, $src, $deps, $ver, $in_footer );

More info here: https://codex.wordpress.org/Function_Reference/wp_enqueue_script